First Light and Reionization Epoch Simulations (FLARES) -- XVIII: the ionising emissivities and hydrogen recombination line properties of early AGN

TL;DR

This paper uses cosmological simulations combined with models to predict the properties of early active galactic nuclei, matching observed H-alpha emission data at high redshift, and offers insights into galaxy formation and black hole activity in the early universe.

Contribution

It introduces a method to predict observable properties of high-redshift AGN from simulations without relying on uncertain SED assumptions, aligning predictions with recent JWST observations.

Findings

Successfully reproduces H-alpha luminosity function at z=5

Predicts ionising photon luminosities of high-redshift AGN

Provides a framework to compare simulations with JWST data

Abstract

One of the most remarkable results from the \emph{James Webb Space Telescope} has been the discovery of a large population of compact sources exhibiting strong broad H$\alpha$ emission, typically interpreted to be low-luminosity broad-line (Type 1) active galactic nuclei (BLAGN). An important question is whether these observations are in tension with galaxy formation models, and if so how? While comparisons have been made using physical properties (i.e.~black hole mass and accretion rate) inferred from observations, these require the use of SED modelling assumptions, or locally inferred scaling relations, which may be unjustified, at least in the distant high-redshift Universe. In this work we take an alternative approach and forward model predictions from the First Light And Reionisation Epoch Simulations (FLARES) suite of cosmological hydrodynamical zoom simulations to predict the observable properties of BLAGN. We achieve this by first coupling \flares\ with the \qsosed\ model to predict the ionising photon luminosities of high-redshift ($z>5$) AGN. To model the observed broad H$\alpha$ emission we then assume a constant conversion factor and covering fraction, and the fraction of AGN that have observable broad-lines. With a reasonable choice of these parameters, \flares\ is able to reproduce observational constraints on the H$\alpha$ luminosity function and equivalent width distribution at $z=5$.